The following description of background art may include insights, discoveries, understandings or disclosures, or associations together with disclosures not known to the relevant art prior to the present invention but provided by the invention. Some such contributions of the invention may be specifically pointed out below, whereas other such contributions of the invention will be apparent from their context.
Ropes are typically used in cranes for hoisting cargo, for example containers. Ropes wear during use and their condition needs to be monitored to ensure their safety. Typical faults of ropes include Local Faults (LFs), where single wires are broken on the surface of the rope or inside the rope, and Loss of Metallic Area (LMA), where the diameter of the rope is reduced. A faulty rope may have an increased diameter by the surface of the rope being faulty. The diameter may be increased for example by dirt, a foreign object being attached to the rope, and/or loose strands or wires on the surface of the rope. In another example the rope may be faulty by a foreign object being attached to the rope, whereby the diameter of the rope may be increased.
Typically condition of ropes is checked by measuring each rope at a time over its whole length to determine the condition. Dedicated measurement instruments may be attached to the rope for the duration of the measurement and the maintenance personnel performing the measurements may also inspect the rope visually. After the measurements are performed, the instruments are detached from the rope and a next rope may be inspected. Accordingly, the typical checking of the condition takes time and requires highly qualified experts that are specialized in maintenance of the ropes. The availability of the experts and the measurement instruments to check the ropes of the cranes may also affect the scheduling of the rope maintenance, making the scheduling of the maintenance even more difficult, whereby high operational efficiency is even more difficult to achieve.
Accordingly, condition of the ropes is typically checked manually by instruments that are temporarily installed to the ropes by the service personnel. Typically these instruments check the condition of the rope by magnetically saturating the rope and measuring the magnetic flux inside and outside of the rope. These instruments fit tightly around the monitored rope to allow efficient transfer of the magnetic flux to and from the rope. However, the instruments have to be removed after the measurements have been performed so that the ropes and the crane may be operated for handling payload. If these instruments are not removed from the ropes, the instruments may travel attached to the rope to hoisting machinery and consequently result in seriously damaging the hoisting machinery and even dropping any payload carried by the ropes to the ground.
Consequently, the present instruments require manual work by maintenance personnel which introduces the possibility of human error and on the other hand since the present instruments cannot be used when the payload is being handled, monitoring the condition of the ropes requires scheduling of maintenance during which the crane is not used for handling payload.
US RE40166E discloses a magnetic non-destructive method and apparatus for measurement of cross sectional area and detection of local flaws in ropes. A magnetic flux is created to the rope under test. A base flux flows through the rope between the poles. Some of the magnetic flux is leaked outside of the rope and forms a leakage flux. Accessory inserts are used to enable testing of ropes having various diameters within a predetermined range.
Accessory inserts should fit tightly between the poles and the rope for efficiently conveying the magnetic flux between the rope and the poles and thereby avoiding losses in the magnetic flux. Thereby, the accessory inserts are specific to each rope diameter used.
Ropes exist in various sizes depending on their application area. The cross-sectional diameter of the rope may vary for example due to the construction of the rope, manufacturing material, required strength and requirements posed by the application area of the rope. On the other hand the diameter of the rope may change during its use, for example due to wearing of the rope.
The distance of the poles from the rope under test is changed as the diameter of the rope is changed. The distance of the poles to the rope affects the proportion of the magnetic flux that is leaked outside of the rope and the proportion of the magnetic flux that is carried through the rope. This causes inaccuracies to the testing of the rope by measuring the magnetic flux. Accordingly, the distance of the poles to the rope affects the magnetization of the rope under test, i.e. the amount of magnetic flux through the rope under test. The magnetic flux sees air gaps between poles and the rope under test as resistances, whereby an increase of the air gaps is reflected to a decrease in the amount of magnetic flux that is carried by the rope. As the amount of magnetic flux carried by the rope is decreased by the increased air gaps, the saturation of the rope is also decreased and the saturation of the rope may even be removed. The lower saturation of the rope may cause that defects in the rope are not detected since the low saturation of the rope provides that the proportion of the magnetic flux carried by the rope and leaking outside the rope due to defects is reduced or even negligible, making it difficult or even impossible to detect defects in the rope from the magnetic flux leaking outside of the rope.
On the other hand, if the variance of the rope diameter is compensated by inserts between the poles and the rope under test, the fitting of the inserts takes time which reduces the operational efficiency of the ropes and the crane where the ropes are installed. Moreover, installation of the inserts requires competent personnel at least for the sake of performing the installation securely in locations that may be high above the ground and/or have a danger of high voltage. These personnel may not be available in the same country or even in the same continent. Thereby the use of the ropes may be prevented at least for the sake of security aspects until the competent personnel are on-site to perform the installation. The manual work needed for installing the inserts also introduces a risk of human error. Moreover, attachment of parts such as the inserts that are installed such that they may be later uninstalled, are prone to loosen up unintentionally which introduced the risk of the loosening being undetected and false test results of the rope and an increased need of maintenance.